This invention relates to a method for manufacturing a synthetic resin case having multiple wall construction. In general, parts integrally made from synthetic resin including polyethylene or polypropylene are manufactured by welding synthetic resin because it cannot be bonded with adhesives. One of the prior art is a welding process in which two synthetic resin parts to be jointed are faced opposedly on either side of a steel plate and pressed together to the steel plate, and then said steel plate is heated and quickly removed to complete jointing the parts while the jointing surfaces of the two resin parts are melted. However, this method is not suitable for jointing parts having complex shape. For instance, assuming that a case having vertical multiple walls is manufactured, the method has difficulties because said method cannot be applied to inner jointing walls of the case if any elevation of inner jointing portions are different from that of the outermost jointing portions, and yet even if identical, uniform welding cannot be expected because of longer distance for the plate to be pulled away.